The invention relates to a method of monitoring a vital parameter of a patient by measuring attenuation of light emitted onto tissue of the patient, comprising the following steps: modulating the light with a modulation frequency or/and a modulation code; emitting the modulated light onto the tissue of the patient; collecting light which is transmitted through the tissue or/and which is reflected from the tissue; demodulating the collected light; analyzing the demodulated collected light with regard to interference with ambient light; determining a modulation frequency or/and a modulation code for which interference with the ambient light is minimized or falls under a predefined threshold; and setting the modulation frequency or/and the modulation code for modulating the light according to the determined modulation frequency or/and a modulation code for which interference with the ambient light is minimized or falls under a predefined threshold. In this way a versatile and reliable possibility of monitoring a vital parameter of a patient with a high signal-to-interference ratio is provided.